This invention relates to diffractive color separation and more particularly to a diffraction color separation microlens which is trimodal.
It will be appreciated that color liquid crystal display panels are routinely utilized in laptop computers. However, these displays are not generally sunlight viewable due to the reflection of the sunlight back towards the viewer. In an effort to provide such displays with enough brilliance and enough color saturation to be able to be viewed in direct sunlight, recently a liquid crystal display has been provided with a microlens array in which the colors from a light source are separated out into distinct bands which illuminate the various red, green and blue sub-pixels associated with the display.
The microlens array used for diffractive color separation may be made in accordance with U.S. Pat. No. 5,600,486, incorporated herein by reference, in which there are only a few steps in each diffraction line of the lens. Recently by applying a genetic algorithm, a large number of steps are provided per grating line to improve color separation and saturation. In this improvement over the Gal et al Patent, a microlens array is provided in which each microlens diffraction grating has a large number of steps for each grating line. However, the thickness of such a microlens in one embodiment is on the order of 12 microns. The 12 micron thickness is required due to the microlens design in which all portions of the microlens contribute to the red, green and blue bands of light imaged on the sub-pixels of the color liquid crystal display. Because of the many steps for each diffraction line a deeply etched structure is required. Generating such a deeply etched lens is difficult due to the multiple masks required and extensive process control, as will be appreciated control difficulty is proportional to the etch depth. It is therefore desirable to provide a microlens in which etch depth can be minimized so as to bring down the overall thickness of the lens from a 12 microns thickness to a 4 microns thickness. The result of so doing is to decrease the etching time and increase etch accuracy. Additionally, the ability to limit the etch step depth decreases the number of different masking steps necessary to provide the various steps in the diffraction grating.
Moreover, with the aforementioned microlens arrays it will be appreciated that each microlens is responsible for separating collimated light into three colored bands, each focused on one of the three sub-pixels. What this means is that for sub-pixels which are not the center sub-pixel, namely the red sub-pixel and the blue sub-pixel, light exiting the sub-pixel is not symmetrically distributed around the normal to the face of the display. What this means is that in uncompensated displays the observer must move his or her head to the left or right in order to see the red or blue hue. For the red pixel for instance, if the person""s head is to the left of the normal, the individual may see no color at all or a black region. What this means is that for displays which are not angle corrected, off-normal viewing is impeded.
In the past, in order to correct for the off-normal viewing a properly designed diffuser adjacent the front face of the liquid crystal display is required to distribute the light into the appropriate viewing angle. However the design of such a diffuser is made somewhat complicated by the non-symmetrical light emanating from the red and blue sub-pixels.
By way of further background, U.S. Pat. No. 5,781,257, incorporated herein by reference, describes a flat panel display utilizing the microlenses. U.S. Pat. No. 5,497,269, incorporated herein by reference, describes a dispersive microlens for use in detecting multiple, different wavelengths and for combining a plurality of different, emitted wavelengths. As illustrated in U.S. Pat. Nos. 5,482,800 and 5,310,623, a method for fabricating microlenses utilizing photolithographic techniques is described.
In order to provide for a thinner microlens structure involving significantly less etch step depth and in order to correct for non-symmetrical distribution of light from the non-center sub-pixels, in one embodiment a trimodal lens is provided with a central region which diffracts the light about a normal thereto in the usual red, green, blue diffraction pattern. This is accomplished by the center third of the lens. The left hand third of the lens has specially configured diffraction lines to provide a blue, red and green diffraction pattern and the right hand third has specially configured diffraction lines to provide a green, blue, red diffraction pattern. The result is that for any sub-pixel, its light comes from the lens segment immediately in front of it and two adjacent lens segments such that the cone of illumination is symmetric about a normal to the microlens array and has the viewing direction. This eliminates the dead zones associated with single mode microlenses.
As a result, all light from a sub-pixel exits in a cone symmetrical about this normal, thereby minimizing the viewing angle discrepancy, and permitting a much simplified diffuser for the liquid crystal display.
In addition to solving the viewing angle discrepancies associated with prior microlens diffractive separators, the subject array of microlenses can be fabricated more exactly due to the thinness of the microlens and decrease in depth of the etching steps. The reason is that since a sub-pixel has light from the three closest symmetrically arranged microlens segments, a single segment need not output light at a larger angle, as in the case with a single mode lens. A significant portion of the etch depth is due to the focusing requirement. With the reduced focusing requirement of the trimodal lens, the total etch depth is reduced dramatically.
The iterative algorithm for defining the stepped structure and the fresnel focusing is now described. For purposes of this invention a genetic algorithm is an iterative method to design the surface profile of the color separator by defining several physical features of the surface profile as genes which are allowed to evolve to an optimal solution. For the present purposes the gene involved is the width of each of the optical elements. Other factors include the design wavelength and the focal plane as physical constraints. Each generation of the genetically defined surface function is scored with a testing function. High scoring surfaces are allowed to go on to the next generation, thus to provide a test of the fit of the surfaces to provide an ideal result.
In the present case the widths and heights of all portions of the design including the diffraction grating and the fresnel portion are provided in terms of a set of seed functions. The resulting surface profile is tested with the testing function, with the testing function giving an overall weight to the profile generated. Succeeding generations of the design are based on the initial seeds plus random elements to allow for genetic variation. Iteratively stepping through this process provides a maximal fit for optimal design.
In one embodiment, with physical constraints as to step width and the overall pixel size, a seed set is generated which includes a particular step width and a pattern of step heights for each element. In one embodiment the microlens is to have a 330 micron diameter. This is broken up into 1.2 micron elements. The seed set generates a surface which is tested using standard diffraction grating theory to ascertain the diffraction efficiency and focusing capability. Standard diffraction grating theory is described in a text entitled Optics by Hecht, p. 312-465 in a section entitled Diffraction. The surface profile of the microlens can be described as a transmission phase grating with non-uniform placement of elements. The test function analyzes the phase contribution from each element from first principles of diffraction theory.
The test determines at a given off axis angle what the expected light intensity should be at a given color. The test also convolves the focusing merit figure, so that after a number of iterations in which random variations are added, an optimal solution is finally achieved.
This is accomplished by crossing for instance 5 seeds with each other. This means that the result of one seed is crossed with the result of another seed, with the results being tested. If 5 seeds are crossed one can obtain as many as 25 testable results. These results are tested and the worst 20 are discarded. The results are then crossed again along with a random seed input to permit the genetic change.
Thus, in general a genetic algorithm is one in which as number of seeds are used to create a result. The results are then crossed with each other to obtain multiple results which are then tested. Superior results are selected and crossed with each other along with a random seed factor and the results tested again. In this manner a large number of results can be tested to determine the optimal result.
In summary, a trimodal microlens configuration is provided for the lenses in a microlens array utilized as a diffraction separator for generating separated bands of different color when the microlens array is provided with a collimated light source. To provide the trimodal functionality, each microlens is divided up into three segments, with the center segment providing a red, green, blue diffraction pattern, with the left segment providing a blue, red, green diffraction pattern, and with the right segment providing a green, blue, red diffraction pattern. This pattern is directed towards an adjacent liquid crystal color display in which its sub-pixels are arranged red, green, blue, with the green sub-pixel aligned with the center segment of the corresponding lens. The result of the trimodal lens is an overlapping of illumination for each of the sub-pixels in that the lens segment aligned with the particular colored sub-pixel provides one third of the light, with the other two thirds of the light being provided by adjacent segments of the lenses in the microlens array. The result of the overlapping illumination of the sub-pixels is that the viewing direction is symmetric about a normal to the array which makes the design of the diffuser used with liquid crystal displays quite simple and eliminates off-axis dead zones. The trimodal structure of the microlens also permits fabrication of a thin lens which minimizes the number of steps for a diffraction grating and focusing lens and thus limits the number of etching steps required as well as making the etching process easier to control.